The invention relates to a multicomponent photometer, comprising
(a) a light source emitting a continuum and from which a measuring light beam originates,
(b) a sample vessel in which a sample gas can be introduced and through which the measuring light beam passes,
(c) a plurality of first gas vessels which are filled with different gases looked for in the sample gas,
(d) one or several second gas vessels, each of which is associated with at least one of the first gas vessels and contains a reference gas,
(e) one or several filters, each of which transmits only a limited spectral region about an absorption band of a gas contained in a first gas vessel (blocking filter),
(f) a detector to which the measuring light beam is applied, and
(g) switching means which are arranged to optionally move into the path of rays of the measuring light beam a first gas vessel with an associated blocking filter for providing a measuring path of rays, or a second gas vessel with an associated blocking filter for providing a reference path of rays.
Multicomponent photometers of this type serve to determine the concentration or the partial pressure of a gas looked for in a gas mixture forming the sample gas. The gas looked for in the sample gas absorbs at a certain absorption band. Therefore, after the measuring light beam has passed through the sample vessel, the more the measuring light beam is weakened in the wave range of the absorption band, the higher the partial pressure of the looked-for gas is in the gas mixture of the sample gas. If this weakened measuring light beam then passes through a "gas filter", i.e. one of the first gas vessels which is filled with the gas looked for, the measuring light beam will be further weakened in this wave range. If, however, after switching over, the weakened measuring light beam, instead of passing through the first gas vessel, passes through a second gas vessel which is filled with a reference gas and does not contain the gas looked for, no further weakening will be effected. The smaller the difference between the intensities obtained with the "measuring path of rays" with the first gas vessel and the "reference path of rays" with the second gas vessel, the more the measuring light beam has already been weakened in the sample vessel. If the sample gas does not contain the looked-for gas at all, in the ideal case the measuring light beam is not at all absorbed in the region of said absorption band. The absorption of the concerned wave range in the first gas vessel is very strong in terms of the absolute values of the intensity. Thus, a considerable difference results between the measuring path of rays in which this absorption take place and the reference path of rays in which a second gas vessel is arranged, and the wave lengths of the absorption band of the gas looked for are not absorbed. If, on the other hand, the gas looked for is contained with high partial pressure in the sample gas, the measuring light beam is almost completely absorbed in the wave range of the absorption band. Then it makes no difference in the intensities falling on the detector as to whether the first gas vessel or the second gas vessel is located in the path of rays of the measuring light beam.
It should be noted that the expressions "measuring path of rays" and "reference path of rays" refer herein to one single geometrically unchanged measuring light beam into which only different optical components are inserted.
Because the absorption band of a gas looked for covers only a narrow region of the total spectrum, the absorption in the wave range of this absorption band would make up only a small percentage of the total intensity which falls on the detector. Furthermore, disturbances can be caused in that absorption bands of the gas looked for overlap with absorption bands of other gases in some wave ranges. Therefore, additional filters, "blocking filters", are provided which, in each case, transmit from the continuum only a wave length range about a concerned absorption band of the looked-for gas.
In a multicomponent photometer different "first" gas vessels with different looked-for gases and associated "second" gas vessels with appropriate reference gases can be optionally moved into the path of rays of the measuring light beam. Then, one or another component can be optionally determined in the sample gas.
In such mulitcomponent photometers an associated blocking filter has to be provided for each component to be determined. The wave ranges of the absorption bands of the different gases to be determined are different. Correspondingly, the wave length ranges about the absorption bands which are cut out of the continuum are, in general, also different.
In known multicomponent photometers these blocking filters are mounted on a single filter wheel which also carries the first and the second gas vessels. Then, one blocking filter appropriate for the concerned gas vessel is located in front of each gas vessel. This is mechanically simple but requires one blocking filter each for the first and for the second gas vessels associated with a looked-for gas.
In general, however, the transmittal regions of such blocking filters are wide compared to the width of the absorption bands. Because of this, small variations in the filter characteristics such as those which can be caused by temperature variations, for example, can already lead to disturbing signals which reach the magnitudes of the desired signal. Pairs of filters, with which such variations appear in a corresponding way, can only be obtained, if at all, by expensive selection.
Therefore, it is the object of the invention to provide a multicomponent photometer of the above defined type such that variations of the "blocking filters" do not affect the measurement.